1. Field of the Invention
The present invention relates to a display device having an emissive element which is driven by current (hereinafter referred to as driven-by-current type emissive element) such as an organic electroluminescence (EL) element.
2. Description of Related Art
Electroluminescence (EL) display devices having an EL element, which is a driven-by-current type emissive element, in each pixel are advantageous in that they are self-emissive type, are thin and consume a small amount of power. Therefore, EL display devices have attracted interest and have been studied as potential replacements for devices such as CRT or LCD displays.
In particular, an active matrix type EL display device in which a switching element, such as a thin film transistor (TFT), for individually controlling the EL element is provided in each pixel to thereby control the EL element for each pixel is expected to be able to provide a highly precise display device.
FIG. 1 illustrates a circuit structure corresponding to one pixel portion of a matrix type (including m rows and n columns) EL display device. In the EL display device, on a substrate, a plurality of gate lines GL extend in the row direction and a plurality of data lines DL and power source lines VL extend in the column direction. The region around the area enclosed by the data line DL, the power source line VL, and the gate line GL corresponds to one pixel region in which an organic EL element 50, a switching TFT (first TFT) 10, a driving TFT (second TFT) 20 for the EL element and a storage capacitor Cs are provided.
The first TFT 10 is connected with the gate line GL and the data line DL and becomes on when a gate signal (a selection signal) is applied to the gate electrode of the TFT 10. At this time, a data signal being supplied to the data line is stored in the storage capacitor Cs which is connected between the first TFT 10 and the second TFT 20. A voltage in accordance with the data signal, which has been supplied via the first TFT 10 and is stored in the storage capacitor Cs, is applied to the gate electrode of the second TFT 20, which then supplies a current in accordance with the gate voltage from the power source line VL to the organic EL element 50. By this operation, the organic EL element for each pixel emits light with an emission intensity in accordance with the data signal, this displaying a desired image.
Each of the EL elements in an organic EL display device is a driven-by-current type emissive element which emits light in accordance with a current flowing between an anode and a cathode. Therefore, the power consumption of the panel varies depending on the number of elements which emit light on the panel, and the power consumption as a whole increases as the number of emitting points increases.
However, with the recent increase of electronic devices such as a display of a mobile telephone, for which low power consumption is an essential requirement, in order to use an organic display device as a display of such an electronic device it is necessary to control the power consumption of the display, and particularly to reduce the maximum power consumption. Further, since the organic EL element generates heat by being driven with a current, there is a possibility that the value of the current flowing in the organic EL element will increase even if the voltage at the power source line VL is at a fixed level, which causes further unnecessary power consumption. In view of such a disadvantage, it is highly desired to control an amount of current flowing in the EL element.
The present invention was conceived in view of the aforementioned problems of the prior art and aims to enable control of the maximum power consumption of a display device such as an EL panel.
In order to achieve the above object, in accordance with one aspect of the present invention, there is provided a display device comprising a display section including a plurality of pixels, each pixel having a driven-by-current type emissive element which includes at least an emissive layer between an anode and a cathode; a power source section for generating a current to cause the driven-by-current type emissive element in the display section to emit light; and a current control section for controlling an amount of current to be supplied to the driven-by-current type emissive element in accordance with an amount of current flowing from the power source section to the display section.
A driven-by-current type emissive element such as an electroluminescence element emits light in proportion to a current being supplied. Therefore, as the number of pixels which emit light in the display section increases, a current flowing from the power source to the display section increases, thereby increasing the power consumption of the device. According to the present invention, as the amount of current to be supplied to each driven-by-current type emissive element is controlled in accordance with the amount of current flowing from the power source toward the display section, the current flowing through each element is restricted to an appropriate range in the display section as a whole even when a large number of elements emit light, thereby reducing the maximum power consumption.
In accordance with another aspect of the present invention, in the above display device, the current control section is provided between the power source section and each driven-by-current type emissive element of the display section.
In accordance with still another aspect of the present invention, in the above display device, the current control section reduces a power source voltage to be applied to each driven-by-current type emissive element to thereby decrease an amount of current to be supplied to each driven-by-current type emissive element when the amount of current flowing from the power source section to the display section increases. By reducing the power source voltage applied to the element under the above control, it is possible to reduce a current flowing through the element easily and reliably.
In accordance with a further aspect of the present invention, in addition to, or independent from, the above control, the current control section controls contrast or brightness level of display data to be supplied to the driven-by-current type emissive element.
In accordance with a further aspect of the present invention, the current control section reduces contrast or brightness level of the display data when the amount of current flowing from the power source section to the display section increases.
Each of the driven-by-current type emissive elements emits light when a current in accordance with the display data flows therethrough. Therefore, when the current to be supplied to the display section from the power source section increases, by reducing the contrast or brightness level of the display data, the amount of current flowing through each element is decreased, thereby ensuring reduction of the power consumption in the display section.
The driven-by-current type emissive element may be, for example, an organic electroluminescence element. By controlling the amount of current to be supplied to the organic EL element, it is possible to prevent an excessive current from flowing through this element, which further contributes to prolonged life and improved reliability of the element.
As described above, according to the present invention, the amount of current to be applied to the driven-by-current type emissive element such as each electroluminescence element is controlled in accordance with the amount of current flowing from the power source to the display section, so that the power consumption of the display section as a whole can be restricted within a predetermined range. Further, when a large number of pixels emit light in the display section, it is possible to prevent the display from being too bright and difficult to view by reducing the increased amount of current.